Hydrometallurgical treatment of copper sulphide ores, such as chalcopyrite (CuFeS.sub.2), is problematical because the severe conditions required in a pressure oxidation step for the effective leaching of copper from these ores results in oxidation of the sulphide in the ore to sulphate, resulting in the generation of large amounts of acid which requires expensive neutralization. Attempts have been made to render the sulphide concentrate leachable under relatively milder conditions under which the sulphide would only be oxidized to elemental sulphur and not all the way through to sulphate. These attempts include the pretreatment of the concentrate prior to the pressure oxidation step to render the sulphide concentrate more readily leachable, and the leaching of the concentrate in the presence of chloride ions, such as described in U.S. Pat. No. 4,039,406. In this process, the copper values in the concentrate are transformed into a solid basic copper sulphate from which the copper values must then be subsequently recovered, as described in U.S. Pat. No. 4,338,168. In the process described in U.S. Pat. No. 4,039,406 a significant amount (20-30%) of sulphide in the ore or concentrate is still oxidized to sulphate, resulting in greater oxygen demand during the pressure leach and the generation of sulphuric acid. This is particularly unfavourable for low grade concentrates, where the S/Cu ratio is high.
The present invention provides a process for the hydrometallurgical extraction of copper in which the oxidation of sulphide in the ore or concentrate to sulphate is reduced and which process is capable of treating both high grade and low grade copper ores or concentrates.